1. Field of Invention
The present invention relates to a light guide plate which is supplied with light sideways and deflects the light to output from an emission face, further relating to a surface light source device employing the light guide plate as well as to a liquid crystal display employing the surface light source device for back-lighting or front-lighting.
2. Related Art
A surface light source device of a type comprises a light guide plate having an end face, through which light is introduced, and two major faces (i.e. faces larger than end faces) one of which provides an emission face, being employed for various uses such as back-lighting or front-lighting for a liquid crystal display. Basic performance of surface light source devices of such type greatly depends on light guide plates employed therein.
A basic function of a light guide plate is to change a propagation direction (roughly in parallel with an emission face of the light guide plate) of light introduced into the light guide plate through a side end face so that the light is emitted through the emission face. As known well, a simply transparent light guide plate without any modification is capable of deflecting light little, providing unsatisfactory brightness. Consequently, any means for promoting emission through the emission face is required.
In general, means for promoting emission from a light guide plate relies upon one of the followings or some of them as combined:
(1) Scattering power within the light guide plate (light scattering guide plate);
(2) Emission face (a major face) provided with light diffusibility;
(3) Back face provided with light diffusibility;
(4) Emission face provided with light-refractive unevenness;and
(5) Back face provided with light-refractive unevenness.
Ways based on (1) provide uniform and highly effective emission with ease. However, the emission is subject to have a preferential direction much inclined with respect to a frontal direction. Usually, the inclination is about 60 to 75 degrees to a normal with respect to the emission face.
Accordingly, an element for modifying the inclined direction to the frontal direction (prism sheet) must be arranged. A light diffusion sheet, if alternatively employed, can bring some increase in frontal emission. However, light diffusion involves a loss of light because of divergence toward useless directions.
Ways based on (2) or (3) hardly provide a highly effective emission. The emission is also preferentially directed to oblique directions as in the case of (1). An increase light diffusibility checks the efficiency because of factors such as wide range scattering or absorption by light scattering elements (for example, by a white ink).
Although ways based on (4) are capable of causing light to escape from the emission face with ease, positive direction conversions are less effected. Accordingly, emission with a high efficiency is less expected. In particular, it is not advantageous that they fail to generate light rays which travel from the back face to the emission face.
To the contrary, ways based on (5) positively generate light which travels from a back face to an emission face of a light guide plate, being free from wide range light scattering. Accordingly, the ways are latently capable of effectively generating emission directed to roughly frontal directions.
FIG. 1a to FIG. 1c illustrate examples based on the above (5). Referring to the figures, reference number 1 indicates a light guide plate made of a transparent material such as acrylic resin, the plate having a side end face to provide an incidence face 2. A primary light source L is disposed beside the incidence face 2 to be supplied with light from the primary light source L. One of two major faces 3 and 4 of the light guide plate 1 provides an emission face 3. The other major face (called xe2x80x9cback facexe2x80x9d) is provided with a great number of recesses 5 having a cross section including slopes 5a and 5b. 
The primary light source L emits light which is introduced into the light guide plate 1 through the incidence face 2. Upon encountering a recess, a propagation light within the light guide plate 1 (as represented by G1, G2) is inner-reflected by a slope 5a to be directed to the emission face 3. Inner-incidence angle is denoted by xcex8 and emission beams derived from beams G1, G2 are denoted by G1xe2x80x2, G2xe2x80x2. In other words, the slope 5a, which is relatively near to the incidence face 2 (or primary light source L) compared with the other slope 5b, provides an inner-reflection slope for direction conversion. This effect is sometimes called edge-lighting effect.
The recesses 5 are formed like dots or linear channels. As shown in FIG. 1a to FIG. 1c, formation pitch d, depth h or slope inclination xcfx86 of the recesses 5 is varied depending on distance from the incidence face 2. Such variations prevent brightness on the emission face 3 from varying depending on distance from the incidence face 2.
However, prior arts as shown in FIG. 1a to FIG. 1c are subject to the following problems.
1. Light is hard to reach a region behind the slope 5b as viewed from the incidence face 2. Therefore, a reduction of formation pitch d hardly rises direction conversion efficiency and the emission face 3 is apt to show an unevenness in brightness.
2. Sufficient direction control in a plane parallel to the incidence face 2 is not effected. For instance, if beams G1 and G2 are parallel to the emission face 3 but not perpendicular to the incidence face 2, emission beams G1xe2x80x2 and G2xe2x80x2 will be diverged to the right or left as viewed from the incidence face 2. Actually, there is considerable light components which propagate not perpendicularly with respect to the incidence face 2 within the light guide plate 1. Accordingly, it is difficult to provide an emission to a desirable angle or within a desirable angle range spatially (i.e. in both planes parallel and vertical to the incidence face 2).
3. Light leaking through the back face 4 occurs easily because direction conversion for generating light directed to the emission face 3 relies upon once-occurring-reflection (at slope 5a). That is, the condition for total reflection is broken with ease at the reflection for direction conversion. For instance, if beams G1xe2x80x2 and G2xe2x80x2 are required to be directed to approximately frontal directions, inner-incidence angle xcex8 is set at about 45 degrees. This is roughly the same as the critical angle for an interface between air and acrylic resin which is a typical material. Therefore, a considerable part of light propagating slightly downward leaks through the slope 5a. 
The present inventor proposed a light guide plate and surface light source device/LCD employing the light guide plate, which were disclosed Japanese Patent Application Tokugan-Hei 11-38977. A brief explanation of the proposed technique is as follows, being aided by FIG. 2 and FIGS. 3a, 3b. 
FIG. 2 is a plan view showing an arrangement of a surface light source device as viewed from a back side of a light guide plate arranged therein, the arrangement being disclosed in the above-mentioned patent application.
FIG. 3a is a partially enlarged perspective view of the light guide plate employed in the surface light source device shown in FIG. 2, and FIG. 3b is a partially enlarged view of one of projection-like micro-reflectors formed on a back face of the light guide plate. Note that sizes of micro-reflectors are exaggerated for the sake of explanation.
Referring to FIG. 2, a light guide plate 10 made of a transparent material. The light guide plate 10 has an end face (minor face) to provide an incidence face 12. A back face referenced with numeral 14 is a back face provided by one of major faces. The other major faces provides an emission face (See FIG. 3a). The light guide plate 10 has right and left side end faces (minor faces) 15 and 16.
A rod-like primary light source (cold cathode lamp) L is disposed along the incidence face 12 which is supplied with light from the light source. Both ends of the cold cathode lamp L are electrode portions EL1 and EL2 between which a light emitting portion extends with a length somewhat smaller than that of the incidence face 12. Such a design is often employed in order to avoid the electrode portions EL1, EL2 from sticking out.
According to a basic feature of the technique disclosed in the above patent application, a great number of projections 20 are formed on the back face 14.
The primary light source L emits light which is introduced into the light guide plate 10 through the incidence end face 12. An inner propagation light travels within the light guide plate 10 and is reflected generally twice when entering into one of the micro-reflectors 20, with the result that a light directed to the emission face 13 is produced. That is, the micro-reflectors function as xe2x80x9cdirection-conversion means for converting an input light into an inner output lightxe2x80x9d
As shown in FIGS. 3a and 3b, each of the micro-reflectors 20 is configurated as to be projected from a general plane (level plane) representative of the back face 14. The illustrated micro-reflector 20 has a shape like a projection having six faces 21, 22, 23, 24, 27 and 28.
The faces 21 and 22 provide a guiding portion to effect a smooth light input for direction-conversion. The faces 21 and 22 meet each other at a ridge portion 26. On the other hand, the faces 23 and 24 effect reflections twice for direction-conversion, producing an inner output light. The faces 23 and 24 meet each other at a ridge portion 25. The faces 27 and 28 are side walls limiting width of the micro-reflector.
Orientation of each micro-reflector is represented by an extending direction of the micro-reflector. In the illustrated example, the ridges 25 and 26 have a straight-projection-line provided by xe2x80x9cprojecting them onto the general plane representative of the back face 14xe2x80x9d. Arraying of the micro-reflectors is designed so that they align to a direction corresponding to light coming direction in order to rise input efficiency and direction-conversion efficiency.
A great part of input light represented by beams H1, H2 is incident to the incidence face 12 along a direction approximately perpendicular to the incidence face 12. However, light that is actually inputted into the projections is not precisely parallel to the general plane of the back face 14 but progresses somewhat downward (so as to approach the back face 14).
Light that progresses precisely parallel to the general plane of the back face 14 or approaches the emission face 13 advances deep without being inputted to projections 20. Consequently, the projections 20 do not obstruct light advancing and give no region little light reaches, thereby effecting contrary to recesses (See FIG. 1).
Viewing from the standpoint of the beams H1 and H2, the reflection faces 23 and 24 of the conversion output portion configurate a valley getting tapered forward. The ridge 25 corresponds to a bottom of the valley. The valley gets narrower and shallower according to distance from the guide portion. Therefore, a great part of light H1 and H2 entering the valley via the guide portion is inner-reflected by one of the reflection faces 23 and 24, and then inner-reflected again by the other faces 24 or 23.
As a result, a light propagation direction is converted twice three-dimensionally to produce inner output light J1, J2 directed to the emission face 13. The inner output light J1, J2 produce in such a way is emitted from the emission face 13 and used for illuminating an object such as LCD panel. Various variations of arrangement and orientation of the micro-reflectors 20 are allowed. The example shown in FIG. 2 is subject to the following rules.
1. Formation density (covering rate) tends to increase according to distance from the incidence face 12. This prevents brightness on an emission face from varying depending on distance from the incidence face 12.
2. Micro-reflectors 20 are arranged in corner areas A, B near to the electrode portions EL1, EL2 at a specially large density. This prevents, together with orientation of the following item 3, prevents dark areas corresponding to the areas A, B from emerging on the emission face.
3. Micro-reflectors 20 are orientated so as to be approximately vertical to the incidence face 12 almost over the back face 14, with their guide portions being directed to the incidence face 12. In other words, each micro-reflector 20 is orientated so that its conversion output portion has a ridge 25 which extends approximately at the right angle with respect to the incidence face 12.
4. In the corner areas A, B, micro-reflectors 20 are obliquely orientated with respect to incidence face 12, with guide portions being directed to the light emitting portion of the cold cathode lamp L. This causes these micro-reflectors 20 to be orientated corresponding to light coming directions, thereby rising direction conversion efficiency.
5. In both side edge portions 15, 16 except the corner areas A and B, micro-reflectors 20 are orientated so as to be inclined at small angles with respect to the incidence face 12, with guide portions being directed to the light emitting portion of the cold cathode tube L. This causes these micro-reflectors 20 to be orientated corresponding to light coming directions, as the above item 4, thereby rising direction conversion efficiency.
If conversion output portions (directions of inner reflection faces 23 and 24) of micro-reflectors 20 located in a certain range from both side end faces 15 and 16 are designed the so that an inner output light is inclined toward a center portion of the light guide plate 10, an emission with converging property is produced.
6. Micro-reflector arrangement does not have a strong regularity such that many micro-reflectors 20 align along a straight line. This makes the micro-reflectors 20 more inconspicuous. And besides, if incorporated in a liquid crystal display, the micro-reflectors can avoid from bringing Moire fringes which would be caused by an overlapping relation with a matrix-like electrode arrangement.
It is possible to heighten the performance of a light guide plate and surface light source device/LCD employing the light guide plate, which were disclosed in the above propose, by adding contrivances as above.
However, the above-proposed technique remains a problem unsolved. That is, the proposed technique, if applied, a fine unevenness in brightness appears on the emission face of the light guide plate 10 corresponding to size and arrangement pitch of the micro-reflectors 20. This gives a viewer a non-smooth visual feeling (a feeling of glaring).
This problem is supposed to arise due to a fact that a roughly almost of the inner output light of the micro-reflectors 20 escapes and is emitted from the emission face at the first chance with ease, as mentioned with referring to FIGS. 3a and 3b. In this Specification, such an escaping (light) at the first chance is called xe2x80x9cdirect escaping (light)xe2x80x9d
Needless to say, such direct escaping occurs generally corresponding to positions of the micro-reflectors 20. On the other hand, an efficient emission can not be expected in a blank region (a flat region on the flat back face 14) without micro-reflectors 20 among the micro-reflectors. As a result, a fine unevenness in brightness appears on the emission face.
In the instant Specification, the term xe2x80x9cindirect escapingxe2x80x9d or xe2x80x9cindirect escaping lightxe2x80x9d means a phenomena or escaping light which occurs or generates at second or later chances after being inner-reflected by the emission face. Simply saying, if the direct escaping light is produced to much as compared with the indirect escaping light, a fine unevenness in brightness will appear.
This problem will be relaxed to a degree by arraying the micro-reflectors 20 at a high density. However, arraying density is subject to a practical limit.
The present invention aims to overcome the above-mentioned problem of the proposed technique. Accordingly, an object of the present invention is to provide a light guide plate which is improved not only as to have a superior direction-conversion function for light introduced sideways but also as to hardly show a fine unevenness in brightness on an emission face.
Another object of the present invention is to provide a surface light source device which is improved as to produce an output illumination not only with a high efficiency but also with a high-quality involving little fine unevenness in brightness by means of the improved light guide plate. Still another object of the present invention is to provide a liquid crystal display which gives a high-quality display screen by applying the improved surface light source device to an arrangement for lighting a LCD panel.
The present invention is according to a basic idea (as proposed in the above patent application) that inner reflections caused twice at inner surfaces of a micro-reflector is a direction-conversion process, and, further to the basic idea, the present invention employs a great number of projection rows formed on an emission face as to cause the emission to have diversified traveling histories, thereby solving the problem.
In the first place, the present invention improves a light guide plate comprising two major faces to provide an emission face and a back face, and an incidence end face for introducing light.
According to a feature of the present invention, the back face of the light guide plate is provided with a great number of projection-like micro-reflectors for direction-conversion of light, each of which includes a guiding portion and a conversion output portion that includes a ridge portion and a pair of first and second reflection surfaces formed on both sides of the ridge portion respectively as to be inclined with respect to a general plane representative of the back face.
And the ridge portion and the first and second reflection surfaces form a valley in the micro-reflector, the valley tending to get narrower and shallower as being distant from the guiding portion.
Through this, an inner input light reaching the valley via the guiding portion is reflected by one of the first and second reflection surfaces and is further reflected by the other of the first and second reflection surfaces as to produce an inner output light directed to the emission face.
On the other hand, the emission face is provided with a great number of projection rows running approximately at right angles with respect to the incidence end face. This configuration causes some of the inner output light to escape through the emission face and the other to be inner-reflected.
The first and second reflection surfaces are inclined with respect to the general plane representative of the back face preferably at inclination angles different from each other, respectively. The ridge portion extends in a direction which may varies depending on location on the back face. A directional distribution of the emission from the emission face can be controlled depending on the inclinations of the first and second reflection surfaces and an extending-direction distribution of the ridges.
The present invention provides an improved surface light source device which employs the above-mentioned light guide plate. The present invention improves a surface light source device comprising at least one primary light source and a light guide plate having two major faces to provide an emission face and a back face, and an incidence end face for introducing light from the primary light source.
Corresponding to the above-mentioned features of the light guide plate, the back face of the light guide plate is provided with a great number of projection-like micro-reflectors for direction-conversion of light. Each of the micro-reflectors includes a guiding portion and a conversion output portion provided with a ridge portion and a pair of first and second reflection surfaces formed on both sides of the ridge portion respectively.
The ridge portion and the first and second reflection surfaces form a valley in the micro-reflector, the valley being configurated as to tend to get narrower and shallower as being distant from the guiding portion.
Through this, an inner input light reaching the valley via the guiding portion is reflected by one of the first and second reflection surfaces and is further reflected by the other of the first and second reflection surfaces as to produce an inner output light directed to the emission face.
On the other hand, the emission face is provided with a great number of projection rows running approximately at right angles with respect to the incidence end face. This configuration causes some of the inner output light to escape through the emission face and the other to be inner-reflected.
Through this function, direct escaping of the inner output light is controlled. A remarkable large part of the inner-reflected light, which has failed to escape, can have a chance of indirect escaping after travelling along various paths. There is only a slight correspondence between the locations of the micro-reflectors and the positions at which the indirect escaping occurs, with the result that the above-described fine unevenness in brightness renders inconspicuous.
It is preferable to dispose a reflection member along the back face of the light guide plate in order to increase the indirectly escaping light. The reflection member reflects and returns the light, which has been inner-reflected at the projection rows and leaked through the back face, to the light guide plate, thereby giving the light chances of indirect escaping.
Each conversion output portion may has a ridge extending in a direction which varies depending on location on the back face. as to approximately accord with a light coming direction. A directional distribution of the emission from the emission face may be controlled depending on the extending direction distribution of ridge.
Each ridge portion of a conversion output portion may extends in a direction which varies depending on location on the back face as to approximately accord with a light coming direction. Alternatively, Each ridge portion of a conversion output portion may extends in a direction which varies depending on location on the back face as to be inclined at a small angle with respect to a light coming direction.
The surface light source device improved as above may be adopted as a surface light source device for a backlighting-type LCD having a LCD panel illuminated from the back side or for a frontlighting-type LCD having a LCD panel illuminated from the front side (i.e. from the viewer""s side). In these cases, the performance of the surface light source device is reflected on that of the LCD. Thus, the LCD in accordance with the present invention has a display screen which looks well luminous.